The vital index, the annual number of births per 100 deaths, is a simple measure but can often be eye-opening. In fact, the measure provides a valuable insight into population dynamics. A population's composition and size (and growth rate) is influenced by the number of births and deaths (the difference being 'natural increase' (or decline)) and migration. In Tasmania's case, migration includes both international migration and interstate migration. Tasmania's vital index (below chart) is a result of a number of factors: the total fertility rate (TFR) and its historic downward trend (and recent increase); the proportion of the population in the older ages; the number of young people who have moved into childbearing ages; and the effect of immigration, which normally consists of workers and their families who are themselves in the childbearing ages. In Tasmania's case net interstate migration in the childbearing ages has historically been negative, while net international migration in the childbearing ages has been positive. However, interstate and international migrants have differing fertility patterns.

As the Tasmanian population continues to age at a faster rate than the rest of Australia (as a result of interstate migration patterns), it is likely that the state's vital index will continue it's (historic) downward trajectory. In the event that Tasmania's 'natural increase' becomes 'natural decline', any growth in the size of the population will be dependent on net increases in migration, which also has the potential to reverse the natural decline - subject to the age profile of the migrants.